The present invention relates to an apparatus for the gas coating of particles suspended in a fluidized bed. More particularly, the invention relates to such an apparatus wherein the particles, for example, nuclear fuel particles, are coated under high temperature conditions.
It is well known in the prior art to employ coatings of pyrolytic carbon or metallic carbides, for example, to provide protection for nuclear fuel particles of a type used in nuclear reactors. The fuel particles are small, for example, on the order of 500 microns, and may be formed from a suitable fissile and/or fertile material, such as uranium, plutonium, thorium or suitable compounds of such materials.
Within a nuclear reactor, the nuclear fuel particles are exposed to conditions of high temperature and severe irradiation over prolonged periods of operation. In order to assure continued effectiveness within such an environment over long periods of time, it has become common to coat the fuel particles with an impermeable material in order to retain gaseous and metallic fission products within the confines of the individual particles.
Pyrolytic carbon and metallic carbide are specific examples of materials composing such coatings for nuclear fuel particles. The coatings may be applied within a high temperature coating chamber through the introduction of a reactant gas having as a substantial component, or consisting entirely of, a suitable hydrocarbon such as acetylene, propylene, propane or methane.
Examples of fuel particles provided with such coatings are disclosed and set forth for example in U.S. Pat. No. 3,325,363 issued June 13, 1967 to Goeddel et al.; U.S. Pat. No. 3,298,921 issued Jan. 17, 1968 to Bokros et al.; U.S. Pat. No. 3,361,638 issued Jan. 2, 1968 to Bokros et al.; and U.S. Pat. No. 3,649,452 issued Mar. 14, 1972 to Chin et al.
A preferred method for coating nuclear fuel particles with a suitable material such as pyrolytic carbon or metallic carbide comprises the deposition of the desired substance through the high temperature decomposition of the gaseous hydrocarbons of the type noted above. When the particles being coated are relatively small, the coating operation may be efficiently carried out with the particles suspended in the form of a fluidized bed within a high temperature coating chamber. Levitation or suspension of the particles within the fluidized bed is commonly achieved through the controlled introduction of a hydrocarbon gas, an inert carrier gas or a combination thereof, beneath the particle bed. Most commonly an inert carrier gas is employed for this purpose and may comprise argon, helium, nitrogen or hydrogen for example.
Within a preferred configuration for such a coating chamber, the coating chamber base is formed from a plate preferably in the form of an inverted conical member which is porous or otherwise provided with means for introducing the carrier or levitating gas beneath the particle bed.
Within such a coating chamber, the small nuclear fuel particles tend to be suspended within the fluid bed under generally isothermal conditions. The reactant gas being introduced into the high temperatures of the coating chamber is decomposed and results in the deposition of the coating material upon the particles. The various conditions for carrying out such a coating operation are well known in the prior art and include temperature ranges within the coating chamber as well as the rates and pressures under which both the reactant and levitating gases are introduced into the chamber and the duration of the coating operation.
The operation of such high temperature gas coating operations for fluidized beds of particles, carried out in accordance with the prior art, has encountered numerous problems. For example, because of the decomposition of the reactant gas within the high temperature environment, there tends to be substantial deposition or buildup of carbonaceous material upon internal surfaces of the chamber. Such carbon buildup is a particular problem where it tends to interfere with the proper introduction of levitating gases for maintaining the fluidized bed of particles or where it interferes with the introduction of additional reactant gas for carrying out the coating operation. For example, where a porous plate is employed to form the base of the coating chamber and the levitating gases are introduced therethrough, the deposition of carbonaceous material upon the upper surfaces of the plate tends to interfere with the flow of the levitating gas and thus disrupts uniform fluidization of the particle bed. In addition, such carbon buildup has been found to interfere with the unloading of batches of coated particles from the chamber.
Another problem concerns the batch size of particles which may be coated during a single operation within the chamber and the related requirement for assuring that the coated particles have a generally spherical configuration. This requirement is particularly important since facets or flat areas may tend to be developed upon the particle surfaces during coating. Faceting is undesirable since it limits structural integrity of the particle coating especially under severe irradiation conditions through the development of local stresses and anisotropic zones.
Another general problem area relates to efficiency of the coating operation. Three particularly important factors affecting efficiency include the batch size of particles to be coated at one time, the problem of rapidly unloading coated particles from the coating chamber to prepare the chamber for receiving a subsequent particle batch and the amount of maintenance necessary between coating runs. Such maintenance primarily involves the removal of coating material from internal components of the coating apparatus.
Two common techniques for unloading the coating chamber include the vacuum removal of the coated particles through a vacuum probe and the forming of an unloading port in a lower portion of the chamber to permit gravity flow of the coated particles. Vacuum removal of the particles is generally undesirable for various reasons. Initially, the vacuum system must withstand the coating chamber temperatures through the use of special materials and/or cooling of the vacuum equipment or particles. Additionally, operation of the vacuum system is awkward since a wand or probe must be lowered through the substantial depth of the coating chamber. Thus, it is difficult to assure complete unloading of coated particles from the chamber.
At the same time, gravity unloading of the particles has been a problem in the past because of the difficulty of achieving rapid flow of the particles through a suitable unloading port.
Finally, the construction of coating chambers has been relatively complex in the past because of the need for supplying the levitating gas to the chamber and supplying the reactant gas to the coating chamber without decomposition as well as providing a means for rapidly and efficiently unloading coated particles from the chamber.
Accordingly, there has been found to remain a substantial need for an improved method and apparatus for the gas coating of particles suspended in a fluidized bed.